&lt;I&gt;Background.&lt;/I&gt; Co-morbidity between substance abuse and psychotic disorders is high. Few studies have examined the relationship between first-episode psychosis and substance abuse. Several questions emerge from this common relationship and many of them remain unanswered.
&lt;br&gt;&lt;I&gt;Objectives.&lt;/I&gt; To determine the effect of substance abuse on psychosis in terms of onset, duration, severity of symptoms, use of medication and outcome.
&lt;br&gt;&lt;I&gt;Method.&lt;/I&gt; Thirty-three subjects with first-episode psychosis, as well as primary caregivers, were interviewed regarding substance abuse and its relation to illness. Thirty-six control subjects were also interviewed.
&lt;br&gt;&lt;I&gt;Results.&lt;/I&gt; Twenty-seven per cent of subjects abused substances in the 3 months before onset of illness, and 77.8% of the abusers were male. Subjects in the first-episode psychosis group were more likely to choose cannabis as their substance of abuse than controls. They also started abusing substances at a younger age than controls. Subjects with first-episode psychosis who abused substances presented at an earlier age than non-abusers. Substances affected symptoms at baseline presentation.
&lt;br&gt;&lt;I&gt;Conclusions.&lt;/I&gt; Substance abuse has a significant impact on firstonset psychosis as far as age of onset and symptom severity are concerned. Subjects with an underlying vulnerability to psychosis seem to start abusing substances at an earlier age than the general population. Males are more likely to abuse substances than females.